1. Field of the Invention
The present invention relates to auto-zeroed analog-to-digital converters and amplifiers therefor, and relates particularly to a voltage offset circuit for a low noise auto-zeroed amplifier. Voltage offset circuits are particularly useful for annulling system offsets arising from sources being measured.
2. Description of the Prior Art
Almost all currently manufactured meters include means to set the indicator to a zero level. Such means are desirable to annul either offset error inherent in the meter itself or arising from the source being measured. In analog meters the zero adjustment typically comprises mechanical adjusting means, and in digital meters the zeroing feature takes the form of a circuit which injects a selectable or an adjustable voltage at some point in the circuit. For digital meters which utilize an analog-to-digital converter preceded by an amplifier, such voltage offset circuits are typically placed at the input of the amplifier feeding the analog-to-digital converter. In current analog-to-digital converter models an auto-zero function is employed for permitting an initial zeroing phase to precede each measurement phase of the conversion cycle. The auto-zero phase within the analog-to-digital converter is used primarily for annulling voltage errors on the integration amplifier, buffer amplifier and comparator within the analog-to-digital converter itself. An auto-zero signal is, however, also made available as an output signal for use in circuits external to the analog-to-digital converter. A typical auto-zero analog-to-digital converter, for example, is the dual slope converter comprising a pair of integrating circuits ICL 8068 and ICL 7103, manufactured by Intersil.
In utilizing offset circuits for the purpose of annulling offset errors arising from the source being measured, the offset correcting voltage is frequently required to be on the order of microvolts and is therefore difficult to generate accurately in the presence of such commonplace corrupting influences as unintentional thermocouple-type voltages, power supply noise and stray leakage effects.